The chelating agent nitrilotriacetic acid (NTA) is often used to purify recombinant proteins. The complexation of NTA with divalent metal ions (Ni2+, Cu2+, etc.) increases its affinity for polyhistidine-tagged (e.g., His6-tag) molecules, thereby enabling noncovalent binding to the protein tag.

Additionally, this property can be used to form DNA-protein conjugates. For this purpose, NTA or Tris-NTA are clicked onto the DBCO-modified 5'- or 3'- end of an oligo via an azide. Tris-NTA enables a particularly stable noncovalent bond to His6-tags.

Besides specificity and stability, the bond between NTA and His6-tags is also reversible. It can be easily broken under mild conditions by adding competing chelating agents (e.g., imidazole, EDTA).

Furthermore, the Tris-NTA modification allows for clear stoichiometric labeling of His6-tagged proteins.

Literature:

1. Conjugation of DNA with protein using His-tag chemistry and its application to the aptamer-based detection system. Shimada J, Maruyama T, Hosogi T, Tominaga J, Kamiya N, Goto M; Biotechnol Lett. (2008), 30:2001-6. doi: 10.1007/s10529-008-9784-4.

2. Oligohis-tags: mechanisms of binding to Ni2+-NTA surfaces. Knecht S, Ricklin D, Eberle AN, Ernst B; J Mol Recognit. (2009), 22(4):270-9. doi: 10.1002/jmr.941.

3. DNA-enzyme conjugate with a weak inhibitor that can specifically detect thrombin in a homogeneous medium. Shimada J, Maruyama T, Kitaoka M, Kamiya N, Goto M; Anal Biochem. (2011), 414(1):103-8. doi: 10.1016/j.ab.2011.02.035.

4. High-affinity adaptors for switchable recognition of histidine-tagged proteins. Lata S, Reichel A, Brock R, Tampé R, Piehler J; JACS. (2005), 127:10205-15. doi: 10.1021/ja050690c.

5. A facile method for reversibly linking a recombinant protein to DNA. Goodman RP, Erben CM, Malo J, Wei MH, McKee ML, Kapanidis AN, Turberfield AJ; ChemBioChem. (2009), 10:1551-7. doi: 10.1002/cbic.200900165.